how to get m16 without completing game (temporary) hint for Black

In the mission Tiviz Asylum when you get to the bit after the beggining of the level get through that and play till you get to the part when you see a guy with a shield for the first time kill him and all the others then go to the last door to your left once inside shoot at the wall then go and pick it up keep it in your inventory though because there is ammo for it later on in the level

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